Controlled cooling of cobalt-rare earth magnetic alloys

ABSTRACT

Cobalt-rare earth magnets having an improved coercive force are prepared by cooling the cobalt-rare earth alloy from sintering temperature to about 900*C in an argon, helium, or other inert atmosphere and quenching the alloy in a liquid having a coefficient of thermal conductivity of less than 0.155 Btu/hr. (ft.2) (*F./ft.) at 50* to 86*F. In a preferred embodiment the alloy is reheated to about 1,100*C and cooled to about 900*C in an inert atmosphere prior to the immersion step.

' United States Patent 1191 Doser 1 Oct. 1, 1974 15 1 CONTROLLED COOLING OF 3,684,593 8/1972 Benz et al 148/102 COBALT RARE EARTH MAGNETIC 3,755,007 8/1973 Benz et a1. 148/101 ALLOYS OTHER PUBLICATIONS [75] Inventor: Manfred Doser, Edmore, Mich.

[73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: May 24, 1973 [21] Appl. No.: 363,625

[52] US. Cl 148/101, l48/3l.57, 148/102,

[51] Int. Cl. H011 l/02 [58] Field of Search 148/101, 103, 105, 31.57, 148/27, 28, 102; 75/227 [56] References Cited UNITED STATES PATENTS Ray et al l48/3l.57

ASM, Metals Handbook; Cleveland, 1939, pp. 329, (Quenching Media).

Primary Examiner-Walter R. Satterfield [57] ABSTRACT Cobalt-rare earth magnets having an improved coercive force are prepared by cooling the cobalt-rare earth alloy from sintering temperature to about 900C in an argon, helium, or other inert atmosphere and quenching the alloy in a liquid having a coefficient of thermal conductivity of less than 0.155 Btu/hr. (ft?) (F./ft.) at 50 to 86F. In a preferred embodiment the alloy is reheated to about 1,100C and cooled to about 900C in an inert atmosphere prior to the immersion step.

4 Claims, No Drawings CONTROLLED COOLING OF COBALT-RARE EARTH MAGNETIC ALLOYS BACKGROUND OF THE INVENTION The invention herein described was made in the course of, or under, a contract, or subcontract thereunder, with the United States Air Force.

Intermetallic alloys formed of rare earth elements 57-72, including mixtures thereof, hereinafter sometimes referred to by the abbreviation RE or R, and cobalt have been shown to have very interesting magnetic properties. Typical of such intermetallic compounds are Co RE, Co RE, and Co RE Of the rare earth elements samarium in particular has produced magnets having a very high energy product BH In order to maximize the desired magnetic properties in cobalt-rare earth magnets, care must be exercised in the preparation of the magnets. Processes used in the preparation of cobalt-rare earth magnets are disclosed in Benz U.S. Pats. Nos. 3,655,463 and 3,655,464 which are assigned to the same assignee as the present application and incorporated herein by reference. In accordance with these patents, sintered cobalt-rare earth metallic products which can be magnetized to form permanent magnets are formed of a base CoR alloy and an additive CoR alloy in which the base CoR alloy is a solid at sintering temperature or at least partly liquid. The base and additive alloys are each used in an amount to form a mixture which has a cobalt and rare earth metal content substantially corresponding to that of the final sintered product. The mixture is pressed into compacts and sintered at a temperature to produce a final product which has a phase composition lying outside the cobalt-rare earth single phase on the rare earth richer side. Thus, the final sintered product contains a major amount of cobalt-rare earth solid intermetallic phase and up to about 35 percent by weight of the product of a second solid CoR intermetallic phase which is richer in rare earth content than the cobaltrare earth phase.

Other inventions directed to cobalt-rare earth magnets are disclosed in Martin U.S. Pats. Nos. 3,682,714 and 3,682,715 as well as in Benz and Martin US. Pat. No. 3,682,716. These patents are assigned to the same assignee as the present application and are incorporated herein by reference. These patents are directed to the preparation of cobalt-rare earth magnets in which the rare earth component consists of samarium and praseodymium, samarium and lanthanum, and samarium and cerium mischmetal.

SUMMARY OF THE INVENTION The present invention is directed toward maximizing the magnetic properties of a cobalt-rare earth intermetallic alloy by controlling its rate of cooling from its sintering'temperature. The sintering temperature is normally about l,lC for Co Sm. Other rare earth alloy systems sinter at 1,050C to l,l70C. The alloy is allowed to cool in an inert atmosphere from its sintering temperature to a temperature within the range of 825C to 975C. The remainder of the cooling cycle is achieved by immersing the alloy in an inert liquid having a coefficient of thermal conductivity of less than 0.155 Btu/hr. (ft?) (F./ft.) at 50 to 86F. While the liquid can be at room temperature at the time of immersion, improved results are obtained if the liquid is at a temperature of about 200C at the time of immersion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In optimizing the properties of cobalt-rare earth magnets it is necessary to control the rate of cooling after the sintering step. The practice has been to cool after sintering in an argon or helium atmosphere furnace to about 900C at the rate of 1C to 6C per minute. The material is then cooled to room temperature at a more rapid rate by moving the magnets in a cooling chamber having an air cooled or water cooled jacket. The parts continue to be in an argon or helium atmosphere during the rapid cooling cycle. This cycle varies from 10 to 30 minutes depending upon the volume of material being cooled. Frequently the material is recycled by again raising the cut parts to the 1,100C post-sintering temperature, cooling slowly to 900C, and then cooling rapidly as explained above.

Cooling of cobalt-rare earth material by water quenching from 900C has been reported by Jones, Lehman and Smeggil in IEEE Transactions on Magnetics, Vol. MAG-8, No. 3, September 1972, pp. 555-557. Both water quenching and rapid cooling in an argon or helium atmosphere produce distinct kinks in the demagnetization curve which lower the overall properties of the end product.

In accordance with this invention, the material is furnace-cooled from sintering temperature to 900C in the usual manner. The parts are then removed from the furnace and dumped into a liquid having a coefficient of thermal conductivity of less than 0.155 Btu/hr. (ft?) (F/ft.) at to 86F. The parts are allowed to cool to room temperature in the liquid.

The following Table I illustrates the improvement in properties provided by the liquid quench cooling of the present invention:

It is to be noted that the residual induction B, and the energy product BH,,,,,, are not helped by liquid quench cooling. However, the coercive force H,, the intrinsic coercive force H and H are considerably improved. H, is a figure of merit which, by definition, is the H value measured at an induction 10 percent down from the residual induction B,. It is a measurement of how square the demagnetization curve is. Magnets produced by the process of this invention are excellent for applications where high H, values are required. High H, and H values increase the stability of the product.

In order to see if the same effect could be produced with larger size material, the liquid quench process of this invention was applied to magnets having twice the volume of the test bars which provided the data of Table I. The comparable H values are given in Table V II below:

cobalt magnets. For example, it makes no difference whether the rare earth is pure samarium or a mixture TABLE II Sample No. H from Furnace-Cooling H from Liquid Quenching 1 8,373 oersted 9,408 oersted 2 8,191 oersted 8,973 oersted 3 8.806 oersted 9,294 oersted 4 8,251 oersted 8,963 oersted 5 7,492 oersted 9,241 oersted 6 8,289 oersted 9,006 oersted 7 7,916 oersted 8,857 oersted 8 8,165 oersted 9,315 oersted The following Table III lists typical liquids which may be used in the practice of this invention:

such as cerium mischmetal or a mixture consisting of equal parts by weight of samarium and cerium misch- Those liquids having thermal conductivities at the low end of the range are preferred in the practice of this invention. The following Table IV is illustrative of this phenomenon:

TABLE IV Thermal Conductivity of Liquid (Btu/hr. (ft?) F/ft.) at 50F 86F In Table IV there is a gradual decline in H, as the thermal conductivity increases to 0.155 and a sharp falling off thereafter.

Materials which are quenched in a liquid at C will sometimes crack. It is important, however, to quench rapidly from 900C to 300400C to prevent deterioration of the magnetic properties. Since cracking takes place only at the highest quench rates, it is desirable to preheat the cooling liquid to about 200C to reduce the rate of cooling. This does not greatly reduce the rate of cooling during the initial stages of quenching and, therefore, does not adversely affect the magnetic properties of the material being quenched. It does, however, avoid cracking the material and, therefore, is a preferred way to practice the invention.

The improved results achieved by the practice of this invention have applied to different types of rare earthmetal. Improved magnetic properties were achieved in all cases where the material was reheated to a temperature above about 900C for about 30 minutes and then quenched in a liquid within the thermal conductivity range disclosed herein.

While the invention has been described with reference to certain specific embodiments, it is understood that there may be variations which fall within the proper scope of the invention. Accordingly, the invention should be limited in scope only as may be necessitated by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows:

1. The method of improving the magnetic properties of a sintered cobalt-rare earth intermetallic alloy after completion of sintering which comprises:

reducing the temperature of the intermetallic alloy in an inert atmosphere from its sintering temperature to a temperature within the range of 825C to 975C;

and immersing the alloy in an inert liquid at about room temperature, said liquid having a coefficient of thermal conductivity of less than 0.155 Btu/hr. (ft?) (F/ft.) at 50 to 86F.

2. The method of claim 1 in which the inert liquid is at a temperature of about 200C-300C at the time the alloy is immersed therein.

3. The method of claim 1 in which the alloy is at a temperature of about 900C at the time of immersion.

4. The method of claim 1 in which the alloy is reheated to about 1,100C and cooled to about 900C in an inert atmosphere prior to the immersion step. 

1. THE METHOD OF IMPROVING THE MAGNETIC PROPERTIES OF A SINTERED COBALT-RARE EARTH INTERMETALLIC ALLOY AFTER COMPLETION OF SINTERING WHICH COMPRISES: REDUCING THE TEMPERATURE OF THE INTERMETALLIC ALLOY IN AN INERT ATMOSPHERE FROM ITS SINTERING TEMPERATURE TO A TEMPERATURE WITHIN THE RANGE OF 825*C TO 975*C; AND IMMERSING THE ALLOY IN AN INERT LIQUID AT ABOUT ROOM TEMPERTURE, SAID LIQUID HAVING A COEFFICIENT OF THERMAL CONDUCTIVITY OF LESS THAN 0.155 BTU/HR. (FT.2) (*F/FT.)AT 50* TO 86*F.
 2. The method of claim 1 in which the inert liquid is at a temperature of about 200*C-300*C at the time the alloy is immersed therein.
 3. The method of claim 1 in which the alloy is at a temperature of about 900*C at the time of immersion.
 4. The method of claim 1 in which the alloy is reheated to about 1,100*C and cooled to about 900*C in an inert atmosphere prior to the immersion step. 